A plane crash that killed two men on a drug-spotting mission for police was the result of an unseen engine defect, an expert believes.
The witness told a coroner's inquest yesterday that he thought a stuck exhaust valve might have led to the death of civilian pilot Chris Scott, 33,
and Detective Travis Hughes, 37, while they were flying in the Gibbston area near Queenstown in January last year.
Both men died when the Cessna 172 rapidly descended and crashed into the side of a valley.
The accident caused a furore in Parliament when United Future leader Peter Dunne said the plane had been shot down. He later said his comment was an "unfortunate slip of the tongue".
The issue was not mentioned during the inquest yesterday. Police have made a host of changes to their aerial drug-spotting programme since the crash.
But a Transport Accident Investigation Commission probe failed to find a cause for the accident.
The expert witness told the inquest the plane had experienced loss of power a few days before the crash and the cause proved to be a stuck engine valve. This effectively reduced the power of the plane down from four cylinders to three.
"It is not beyond the realms of possibility that the engine could have suffered yet another stuck valve due to carbon build-up," the witness said.
"Chris could well have experienced a partial power loss and a possible thermal down-draught. He may have even been attempting a controlled forced landing on the spur and never made it."
The witness considered Mr Scott safety conscious and a very good pilot in cannabis-spotting operations.
Queenstown coroner Alan Macalister will hear more witnesses today.
